EA Modeling of a Chevy 292 Inline Six, Siamesed Ports

[enigma57]

Thanks to all! Much appreciated! Fascinating history and always informative to go back and explore past practices. I have always felt that at times, new ideas and practices / technology come along and the great majority of engine builders abandon the old and go to the new...... Without having taken some of the old to their ultimate development. That's why I enjoy going back and exploring the older ways of doing things.

Cris, yes, the 292 can certainly benefit from increased compression. We ran into similar issues with the L31 Vortec engine we began with as a basis for the 350 engine that we are putting together for my youngest son's Camaro. We bought it new from a Chevy dealer in the late '90s and its been stored here in my garage ever since.

Like most crate engines, it came with the pistons down in the hole a bit at TDC. And like most truck engines, it came with dish top pistons. Same as your 292 Goodwrench engine. When you do the math by actual measurements, his engine had closer to 7.75:1 actual static compression ratio versus the advertised 9.0:1. Again, same as your 292. This was a crate engine we picked up from Sallee Chrevrolet and it came with a mild CompCams RV type cam and upgraded ZZ4 timing gears and chain that make good low and midrange power with peak power around 5,300 RPMs. His Camaro is a daily driver, not a race car so that fits the bill as far as the basic engine. So, like yourself, I realized that we needed to get actual static comp. ratio up to work with his cam. In his case, we needed to bump static compression to 9.25 - 9.5:1

Over the years, I sold the cast-iron Vortec heads to some circle track guys and the Edelbrock Vortec carburettor intake that came on his engine, as well. So we had a good basic short block to start with last year. Just needed to boost compression ratio slightly to get the most out of it.

Rather than disassemble the short block and replace pistons and have the block zero decked, we ordered new AFR heads he was wanting because they were available with the oddball angle plug layout matching that of L-98 heads and he has a set of never run headers from SLP that were built for the '91 Firehawk that SLP collaborated with GM on and they only clear plugs with the L-98 plug angle. So we had AFR mill the heads and do a spring upgrade when ordering and went with Cometic 0.028" thick MLS head gaskets to boost his compression to a true 9.25:1. That way, we avoided disassembling the short block and DCR is spot on for the cam he is running.

I might add that his engine came from GM with 0.028" thick composition head gaskets having 4.100" bore size. So we picked up compression mainly from milling the heads to reduce chamber volume. When I ordered the Cometic gaskets, I had Cometic make them made for 4.00" standard bore size rather than run off the shelf gaskets with larger bore size, to eliminate the crevice and the little bit of added cc's of volume lost there. And because they are just darn good gaskets and I don't want any sealing issues. By doing it that way, we were able to maintain 0.045" clearance from the non-dished portion of pistons to the flat portions of head outside the actual chambers.

Cris, for your 292, there are pistons having flat tops and pistons having smaller dished area (the 'propane' pistons). And you can gain low end power over that provided by even the OEM cam. But regardless of which way you go, including retaining your present pistons and cam...... It will be important to quantify what you have in order to determine actual (existing) compression ratio and and determine how much you need to increase it to maximize power. So begin by pulling your head and measuring......

* cc your combustion chambers

* cc piston top dishes and any valve reliefs (probably no reliefs in the OEM 292 pistons)

* with piston at TDC, measure distance down in the hole (from metal straight edge placed across bore down to flat part of piston around circumference) (not dished part)

* Measure compressed thickness of stock head gasket and bore width as reference

For 292......
Standard bore will be 3.875"
Standard stroke is 4.120"
Rod length is 6.760"

With that information, you can pin down actual and proposed static compression ratio.

Next, you need to know the intake valve closing point (IVC). This is measured ABDC (after bottom dead center).

With this information added to the above information, you can determine DCR. For a street driven engine on what passes for pump gas now of days, don't exceed 8.00:1 DCR nor go lower than 7.50:1 DCR. I like to run as close to 7.75:1 DCR on the street as possible. This should allow you to run all the ignition timing needed without getting into detonantion so long as you run good gas.

Wallace Racing has some very good calculators......

http://www.wallaceracing.com/Calculators.htm

Static compression ratio......

http://www.wallaceracing.com/cr_test2.php

DCR......

http://www.wallaceracing.com/dynamic-cr.php

With a very mild 292, you can make a worthwhile upgrade from 1.72" / 1.50" valves to 1.84" (305) intakes and 1.60" exhaust and a little bowl work. No need to go to 1.94" intakes with standard bore size and mild cam as you may encounter shrouding.

Of course, the 500 lb. gorilla in the room with these engines is the gosh awful headbolt casting right in the middle of the siamesed headports. You can reduce its size (very carefully) with hand grinding and streamline it using a symmetrical airfoil shape. Or you can remove it altogether and machine the head to accept a short cap screw in its place and blank off the hole left in the port roof above. But that only gets you increased performance worth the effort if you add lump ports (to improve the short turn radius) as a part of headbolt boss removal.

In any case, improved induction and exhaust will help noticeably. As will improved ignition. Tom Langdon developed a nicely done set of cast-iron split manifolds for these engines. As cast, they have 2-1/4" dual outlets. Tom has scaled back and is now concentrating on the older Chevy 6's and flathead MOPAR 6's and he has given his inventory for the later inline 6's to Tom Lowe over at 12-bolt.com . Great guy and very much into the 194/230/250/292 series of Chevy inline 6's.......

https://www.12bolt.com/store/p153/Exh_M ... -292_.html

Regarding induction...... The length of the inline 6 head and the 3 sets of paired, siamesed intake ports make running a single carburettor a challenge, as the center 2 cylinders will get the most air/fuel mixture and the 2 sets of end ports will receive less. This means you have to make sure the end cylinders receive what they need and let the center 2 cylinders run a bit rich with this sort of log manifolding in most cases.

If you are going to improve fuel distribution and gain power from improved efficiency, you will need to look at triple carb manifolds and in a few cases, well designed dual carb manifolds. However, if you are going to run a single, centrally located carb (or TBI), look at the Aussie Speed version. Very well designed and as good as you will find for single carb on these engines......

http://www.aussiespeedshop.com/product/ ... -4-barrel/

Don't let the list price buffalo you. $540 Australian dollars at current exchange rate is US $397 and change. And I have seen these manifolds (new) on US e-Bay for around US $350 or so with free shipping.

As with any wet flow intake, you can only do so much for these engines when running a single carb (or throttle body EFI such as the one you mentioned). If you go that route, run the most efficient intake you can find and it will help a bit.

Can't really comment on the TBI unit you mentioned, as I have not run one. I'm a carb guy. I am putting together a couple carburetted intakes for my engine, a triple and a dual carb setup using Weber DCNF carbs and DCNVH carbs. The triple carb intake is a long runner design I built from 2 modified Brazilian triple sidedraught manifolds. It may present clearance issues with the steering column and brake master cylinder in my '57 Chevy passenger car, though. So if it does, Plan B is to mount a pair of slightly larger Weber DCNF carbs on a Clifford dual carb intake and whittle a hardwood plenum stuffer for the interiour to reduce plenum volume and help direct flow to give better fuel distribution.

Hope this gives you some ideas,

Harry

[strokersix]

Plan B is to mount a pair of slightly larger Weber DCNF carbs on a Clifford dual carb intake and whittle a hardwood plenum stuffer for the interiour to reduce plenum volume and help direct flow to give better fuel distribution.

I'd like to see what this plenum stuffer looks like

Mike

[SchmidtMotorWorks]

I'm not the guy to try the more sophisticated modeling suggested. I am hoping my $200 EA Plus can steer me in the right direction to make these basic decisions.

Regards, Cris

If Siamese ports are not in the model, it won't be able to return data that is relevant to those features.

[Cris]

SchmidtMotorWorks:

Do you think the plenum model has any value? I am not trying to study intake length effects. I will use a stock or near stock intake with the large siamesed ports. The cam will also remain fairly short in duration so little overlap occurs in the siamese ports. But I would like to understand the effects of compression, changes to the cam, and carb CFM. Below is probably the manifold I will use.

Thanks, Cris

Capture.PNG

[Cris]

Enigma57:

Thanks for the data dump. I am using a 12 Bolt HEI on my 230 right now. I also found his website to be a great source of info as I assembled the model for my 292 in Engine Analyzer. (He has both the stock head flow numbers and the lump port numbers.)

I am starting to pull parts off the 292 now. After leakdown I will decide how far I want to dig into this thing.

Regards, Cris

[SchmidtMotorWorks]

SchmidtMotorWorks:

Do you think the plenum model has any value? I am not trying to study intake length effects. I will use a stock or near stock intake with the large siamesed ports. The cam will also remain fairly short in duration so little overlap occurs in the siamese ports. But I would like to understand the effects of compression, changes to the cam, and carb CFM. Below is probably the manifold I will use.

Thanks, Cris

Capture.PNG

Unless the simulation model has parameters that describe the manifold and port merges, it will be meaningless to the manifold.

Frankly, plenum performance is at least as dependent on shape as it is on volume.
It would take years of work to develop a math model to simulate a plenum and runners any better than "what looks good".

If you are not going to build a manifold, then trying a few open spacers is about all you can do.
Might be worth building a plenum box to see what results you get, it might soften the spikes in the plenum.

One thing a manifold shaped like that might be vulnerable to is waves traveling the along runners and crossing the plenum and into the opposing runner.
Dividing the plenum might be worth trying depending on the firing order.

[enigma57]

Plan B is to mount a pair of slightly larger Weber DCNF carbs on a Clifford dual carb intake and whittle a hardwood plenum stuffer for the interiour to reduce plenum volume and help direct flow to give better fuel distribution.

I'd like to see what this plenum stuffer looks like

Mike

Mike, it will be a few months, but I will be building the 3 carb intake first to see how the clearance issues work out. Its that close. Could go either way. If push comes to shove, with a little effort I can relocate the brake master cylinder farther outboard on the firewall and rework /offset the under dash brake pedal linkage to suit. Steering column is another matter altogether, though.

Regarding the Clifford dual carb intake...... They are only offered in Clifford's current product listing as a part of a complete induction system utilizing the Weber 38 DGAS carbs. These are good carbs and the complete Clifford induction setup is very good, as well. However, I am into Weber DCNF and DCNVH carbs. Have worked with them for years and have several sets of them here.

So I called Larry at Clifford. Explained what I was doing and he was very helpful. Was able to buy just the bare dual carb intake. Will make my own carb mounting plates to fit the Weber DCNF carbs to this intake. The Clifford dual carb intake is a very good design as cast. Flows very well. It does have a large plenum area, though. Which is good in the upper RPM ranges, but softens throttle response from off idle to lower mid-range. So I will take a look at the Clifford intake with an eye towards reducing excess plenum volume to improve throttle response. Will configure the plenum stuffer(s) to aid fuel distribution, as well.

Computers are a mystery to me. So will take photos and have wife put them in a zip file I can e-mail you when the plenum stuffer(s) are done. I'm thinking hardwood such as oak would be a good material to make them from. It will take a lot of whittling, sanding and hand fitting to get them the way I want them, though.

Best regards,

Harry

[panic]

The problem is not the CFM, it's the pressure and directional changes that occur during the intake stroke. Blair refers to the exchanged effects as "superposition" because flow is never continuous and uni-directional.

[enigma57]

Enigma57:

Thanks for the data dump. I am using a 12 Bolt HEI on my 230 right now. I also found his website to be a great source of info as I assembled the model for my 292 in Engine Analyzer. (He has both the stock head flow numbers and the lump port numbers.)

I am starting to pull parts off the 292 now. After leakdown I will decide how far I want to dig into this thing.

Regards, Cris

Wish you all the best with your 292, Cris! Look forward to hearing of your progress.

If you run the Offenhauser intake pictured, you might consider making a carb mounting plate that centers the 2 throttle bores of your TBI unit and aligns them with the center of the front to rear run of the intake. Might consider adding a divider beneath carb mounting plate running at right angles to front and rear of engine, effectively dividing the log manifold with cylinders 1,2&3 on one side and cylinders 4,5&6 on the other.

Best regards,

Harry

[Cris]

Harry, I think you are and I are on the same page with how to orient the two barrel. One barrel will be forward, and one rear. (Instead of mounting the carb with one barrel to the right and left.)

I don't have the Offy manifold yet, but I'm not sure whether to remove the four individual holes for the four barrel carb. I could also just mill a bank of two holes into one, making two "slots" that each barrel of the two barrel would feed. I'm looking at this carb adapter:

Capture.PNG

I might have to play around with what works best for the throttle body. Also unsure of whether I will have to heat the intake manifold with the EFI.

Regards, Cris

[Cris]

Another option if I mill out for "slots":

Capture.PNG

[Truckedup]

I built a 292 for a 68 Chevy manual transmission PU truck a few years back...Nothing fancy, got a set of so called 9-1 compression propane pistons.Schnieder mild cam. Had the block decked to get the pistons close enought for quench .Found a thinner head gasket and wound up with .045 quench.A good valve job, stock intake and exhaust manifold from the stock 250 engine. Bigger exhaust pipe and muffler.
It ran very well, much more pull than the 250...Faster than a smogged 305....No detonation on lwer octane pump gas..
The truck was a rare 68 4 wheel drive 4 speed short bed stepside. I listened to guys telling me to keep it original as possible... I should have stuck a period look SBC in it......more for less money...

[panic]

But where's the fun in that!

[enigma57]

Exactly!

HB

[enigma57]

Harry, I think you are and I are on the same page with how to orient the two barrel. One barrel will be forward, and one rear. (Instead of mounting the carb with one barrel to the right and left.)

I don't have the Offy manifold yet, but I'm not sure whether to remove the four individual holes for the four barrel carb. I could also just mill a bank of two holes into one, making two "slots" that each barrel of the two barrel would feed. I'm looking at this carb adapter:

I might have to play around with what works best for the throttle body. Also unsure of whether I will have to heat the intake manifold with the EFI.

Regards, Cris

Cris, of the 2 carb mounting plates you posted, I would suggest going with the Holley plate......



Yes, definitely orient the 2bl carb or TBI as you said "one barrel forward and the other to the rear'.

You may be able to drill the Holley plate to suit and delete the plate provided by Offenhauser with their manifold if your Holley plate will seal well to manifold without the additional plate below it.

On the other hand, so long as hood clearance does not become an issue once you have manifold, carb and air cleaner installed...... Having your carb or TBI positioned a bit higher above plenum floor might aid air/fuel mixture to make the 90 degree change of direction from vertical to horizontal. If you do end up stacking the Holley plate and Offy adapter plate...... You can either slot the Offy adapter plate or open it up.

Me...... All things considered, I would slot the Offy plate if sandwiched between Holley plate and manifold, leaving the center of the Offy plate divided. Definitely leave the Holley plate in its 2-hole configuration, though.

If the carb mounting plate is aluminum and not cast potmetal...... I would also consider welding a hand fitted vertical divider plate or 'wall' to bottom of lower plate extending from bottom of lower plate to floor of plenum so each throttle bore feeds only 3 cylinders. If you do this, hand fit vertical plate so that it 'just' contacts plenum floor without a gasket fitted so you will have clearance when assembled equal to the compressed thickness of your gasket. Do not RTV either the Holley nor the Offy carb mounting plates. Use a good quality gasket even if you have to pick up a sheet of gasket material and cut your own.

In any case, whichever carb mounting plate is bolted directly to the intake manifold...... Take a bur and work the bottom of it to (directionly) 'relieve' the underside so as to aid the air/fuel mix make the turn towards cylinders 1 & 2 and cylinders 5 & 6. Make sort of a short turn radius when shaping these reliefs...... Being careful to leave plenty of gasket surface area to seal plate where it joins intake The closer center cylinders 3 & 4 will get plenty of air/fuel mixture due to their proximity to the single, centrally positioned plenum. No need to 'help' them as you do the cylinders farther away at each end of log manifold.

Yes...... Heat plenum floor with engine coolant using provided under plenum plate. With either a carbor TBI on wet flow intake, manifold heat will help with atomization. TBI will have a coarser spray pattern than most carbs and you will need it even more than if you were running a carburettor. Bring one of your heater hoses to plate under plenum and then continue rearward, turn towards passenger side at firewall and connect to heater core. Support hose every 9" or so to prevent it laying or rubbing on engine. Keep minimum inch and a half air gap to engine if you can.

If you have the vertical space above air cleaner, you can also scrounge up one of the water heated/cooled thermo base plates Ford utilized between carburettor and intake manifold in the early to mid-'60s. Ford made both 2bbl and 4 bbl versions of these base plates. Usually available used on e-bay.

Caveat...... Ford also made similar appearing base plates for later year engines having only vacuum connections and EGR connections. These are not the thermo base plates. Also make sure the water conveyance tubes are not corroded when choosing one of these. Regarding the throttle bore holes in the Ford plates...... If you look closely, there is small 'step' cast into them. Its not casting flash nor is it a defect. Leave it as cast so long as the holes are at least same size as your carb or TBI throttle bores. These 'steps' help atomization and by extension, low speed throttle response, drivability and mileage. If you alter them and make the throttle bore holes nice and smooth like the holes in the Holley carb mounting plate...... Throttle response, drivability and mileage will suffer.

The Ford base plates are same bolt pattern as Holley. If you fit one between Holley plate and intake, just use longer carb mounting studs. Same heater hose routing as with the Offy plenum floor plate from front of engine to connection at heater core except the Ford plate accepts smaller 1/2" coolant hose. You can pick up 3/4" X 1/2" or 5/8" X 1/2" brass reducers with hose barbs on each end to make that work.

Ford thermo base plate, 4bbl version (2bbl version is similar, this one also has provision for vacuum to power brakes)......



Clifford thermo base plate, 4bbl version (no longer made, these were used by Clifford before Jack added plenum heating to his manifold castings, no provision for vacuum)......



This is one of the later year Ford 2bbl versions (has vacuum provision and center divider between throttle bore holes is partially opened up)......



Best regards,

Harry